Altimeter control of servo system



1952 J. H. BROADBENT ETAL 2,580,512

ALTIMETER CONTROL OF SERVO SYSTEM Filed Jan. 25, 1948 ALTIMETER BRIDGE To 5 v (26 22 r g I", I, FOLLOW-UP I SYNOHRO- GENERATOR 25 26 49 2 32 R 2? 3 VARIABLE-RATIO WWI TRANSFORMER 9F214 SERVO 'AlR T49 AMPUHER 48 RECTIFIER 4| t 1 BRIDGE 4O 44 4 no v TRANSFER HYDRAULIC Q VALVE V MOTOR ATTORNEY Patented Jan. 1, 1952 ALTIMETER CONTROL OF SERVO SYSTEM John H. Broadbent, Washington, D. 0., and Henry Rees Mitchell, Takoma Park, Md., assignors to United States of America as represented by the Secretary of the Navy Application January 23, 1948, Serial No. 3,846

3 Claims.

The present invention relates to an improved system for controlling an' amplifier, and other devices if so desired, by means of an altimeter. More specifically, it relates to means and a process whereby a servomotor may be actuated in response to altitude variations; for example, the altitude variations of a guided missile during its flight.

An object of the invention is to provide a simple system, including means responsive to atmos- 2 present specification and the drawing accompanying the same, the single figure of which is a diagram showing the electrical circuits and devices used in such a system.

It will be understood that the specific features disclosed are merely illustrative of one form of circuit and apparatus that may be employed in practising the invention, and that the latter may be embodied also in other forms. I, 'The chief purpose of the invention is to provide automatic means for compensating for the thus the.resistance of its filament constitutes a steadily decreasing air resistance as a missile rises into higher portions of the earths atmosphere, with the result that a setting of a steering vane or air foil that sufiices, at or near sea level, to produce a given path curvature, will give progressively less curvature as the missile passes through air of decreasing density. This is accomplished by causing the air foil to turn through a correcting angle that increases progressively with altitude, so that the effective steering re sponse becomes substantially independent of the altitude of the missile, and the latter does not become sluggish at high altitudes.

The altimeter bridge 52 here shown comprises two similar tubes, each containing a filament,

said filaments preferably having large temperature coefiicients and being connected in two arms of a Wheatstone bridge. One of these tubes I is shown as having an opening 2, placing its inte rior in communication with the outer air, and is thus of the Pirani type. The other tube 3 is'permanently sealed.

Each tube has a filament of very small diameter therein, as shown diagrammatically at 4 and 5, and these filaments, which preferably are iden tical in physical dimensions and materials, as well as in electrical characteristics, constitute the resistors in two arms of the bridge, which are to be balanced against the known resistors 6 and I. forming the other two arms of the bridge.

The principle of operation of this type of altimeter is that the temperature of the electrically-heated filament in the tube I, other things being equal, depends in part on the pressure of the atmosphere, because of variation in the heat conductively carried off thereby upon contact with the heated filament 4. Thus, for a given voltage applied to the filament, producing a heating current therein, the resulting temperature of the filament is a function of the atmospheric pressure, and may be used to measure such pressure, and therefrom to deduce the corresponding altitude.

The temperature variation of the filament 4, of course, produces therein a corresponding resistance variation, which lends itself to electrical measurement by means of the Wheatstone bridge. The tube I, with suitable electrical circuits, constitutes the device well-known as the Pirani gage, suitable primarily for measuring relatively low pressures, such as exist in partial vacua and the like. The temperature of the similarly-dimensioned and located filament 5 of the permanently sealed tube 3, on the contrary, obviously cannot vary with external atmospheric pressure, and

convenient standard for comparison with that of the Pirani tube I, because the similarity of environments of the two filaments automatically compensates for other possible causes of variation.

I six volts being applied to each arm of the bridge.

The voltage and frequency however are arbitrary, any available frequency within reasonable limits being suitable, and the voltage being governed by the nature, dimensions and material of the filaments. Preferablysuch voltage will be main tained that the filaments will be heated to only a relatively low temperature, about 400 C. for example, to avoid excessive radiation losses.

Alternating current is used as a matter of conconsidered in one aspect as an impedance-match- 1 ing device between the relatively low-resistance.

bridge diagonal and the relatively high-impedance control grid circuit.

for the control grid, while IT is the grid resistor and is of relatively high resistance; Thesecond:

or screen grid is here maintained at substantially the same alternating potential as the anode, by"

the 200 volt source indicated at terminal 53. It should'be noted -that both thecontrol grid and the screen grid, as well as -the-anode are' supplied with alternating-voltage,- contrary to usual amplifier practice:

The output transformer it hasits-primary winding [9 in the anode-cathode circuit'of-the thermioniotube i; and consequently said' wi-ndingv is traversed-by anarnplified'fluctuatingcur rent, -whereby a corresponding voltage is produced in the secondary winding This winding is. connected to the winding 22 of a follow up synchrpgenerator 23 the rotor ofwhich'is rotated by 'thegsamemotor; eilthat has the'function. of. controlling the setting" of the steering surface, as will'be discussed"; later. Aphasea adjusting capacitor "21' may be connected across d s .2 and .2 a ow Th a b chosen of theproper capacitance to, get the currents in windings .22 and 35 into phase sothat h n sener to s .2 a d .1 m y. coas p ope y; n /p to .-.$h0 ?i .Q 3 har on o the 400 cycle current; resulting from the pentode circuit.

A rectifierbridge 2l; which .co nprisesthe iour rectifiers 3B, 1 9.1; .38.; and '39; connected with .the relative polarities indicated; ,has its. alternating current input diagonal .conneetedtothe. terminals of.. winding Zflby the. c.0nductors.25;and. 2.6.. This is indicated here. as, the .horizontal. diagpnal,...the

vertical diagonal serving as the direct current output side. The rectified. output. passesthrough the, conductors 2 I and is and thedirectl current control winding 29.. of a variable ratio trans former.

This transformer comprises an alternating current input winding. 33, .the. .alooyeementionfid. direct currentcontrol ,yvinding 29 and .an .alter nating current output winding. 3 l all associated with the same. magnetic. core32. This core serves. as the conventional transformer core as to winds ings 3i). and .35, but due.- tov the direct. current winding 29, a unidirectional magnetic ,flux is superimposed. on the alternating flux, sotthat the degree of i saturation varies.

Asis well .lrnowntheeiiiciency of transformation ofa transformer decreases as saturation of'the core thereof is. approached; that is, an

applied primary alternating voltage, in coil 30.

will produce a relatively lower secondary voltage in coil 3dv whena .largedirect current is flowin through windingidthanwhen the direct current is small. Thus .in. efie ct theratio of transformation isno longer. thesimple turns-ratio. but maybe variedby.suitablemhanges ofthe direct current excitation.

carries: the apparatus.

he applied to the present circuit, this means that when the alternating current voltage output of winding 20 increases, a larger rectified current will flow through winding 29, and the transformation ratio of the said variable-ratio transformer anotherybyg a free. gyroscope, so as to respond to.. -.angular-position. of the guided missile which This synchro-generator is fecl'with a constant voltage in its winding 35, hereashownas26avolts. The voltage supplied to the corresponding winding 22 of the follow-up synchro-generator 23 on the other hand varies say from 26 volts downto about 5 volts. Consequently"- the rotation --of '-synchrogenerator. 23 must var-y inversely-asthis-voltage to providethe' sameoutput. "This means that the stiiT-iieSs" of the"; response willyary as about 5 :1, stiiiness beingdefined as-theratio of the error-correcting deflection 01" the steering surface 'to' the error angle of the missileirom its-intended-course.

To get a corresponding effect on the output of the servoamplifienSS thetransformation ratio of the variable-ratio transformer should also vary from about 5 to 1; 'thatis', at maximum theyoltage induced in the Winding '31 should be about five times that applied to the winding 31 while at minimum the input and output voltages should be approximately'equal. :Suchresults are actually attained, with sufliciently close approach for practical purposes;

In the fiight of" a guide'd' missile, has" it rises to greater; heights stheair becomes. less dense so that an angular'setting of. a steering surface" that sufiices to restore it to its coursewhenoperating at sea'level" becomes inadequate when sai;d' su iaqe mn nee on t e air. nd eko s h for arie .T ,co nsate' orithis.

being sometimes substantially. 9D? in. advance. of

the currentin the.- stationary phase-winding. 42, which is energized. directlyiby the power. supply andsometimes substantially behinds aid.=cur.'-v rent, according... to. the directionof the error signal.

A second device isthehydraulic transfer valve M actuated'loy the servomotor through a suitable mechanicalconnection .43. This valve con trols the flow of a; hydraulic pres urefiuid, such' as oil, to one side or other. of the hydraulic motor 41 through a corresponding one of i' the pipes 55 and 46, so that the piston rod 48 of said hydraulic motor will move in afcorresponding direction.

This piston rod. 48 is. connectedmechanically.

by suitable means 491110 one element 23 of the fol ow-up synchro-generator, and is also connected by a suitable mechanical linkage 50 or equivalent means to an air foil 5| or other steering means, to turn the latter when the piston rod 48 moves.

The result is that, whenever an error signal exists, the servo amplifier supplies excitation to the winding 41 which in connection with the continuously excited winding 42 will cause the rotor 49 to turn in the proper direction to correct the error by turning the vane 5| by means of the hydraulic motor etc. At the same time, the follow-up synchro-generator 23 is turned in the proper direction an amount to ooact with the synchro-generator 34 to reduce the error signal to nearly zero, whereupon the vane 5| will come to rest at its proper setting.

The altimeter bridge yields outputs responsive to atmospheric pressure and thus to altitude, which are amplified by the electronic amplifier l5 and eventually rectified by the rectifier bridge 24 and fed to the variable-ratio transformer, with the altitude-responsive results already explained.

It should be noted that the signal circuit, which consists of the secondary windings of the free-gyroscope controlled synchro-generator 34 and the feed-back synchro-generator 23, and the primary winding of the variable-ratio transformer, constitutes a closed circuit, and the stiffness of this circuit is controlled solely by the excitation of the feed-back synchro-generator 23. The variable-ratio transformer increases the gain of the servo-amplifier proportionally with altitude, thereby decreasin the time required for the servo-motor to accelerate and position the control surface at the requisite angle. When the voltages induced in the two synchro-generators are equal and opposite, no current will flow through the winding 30, no output will be produced by the amplifier 33, and the servo-motor and all the control and other elements actuated thereby will be brought to rest, as then positioned.

What is claimed is:

1. A servo system to be carried by a guided missile, comprising means for producing an alternating voltage whose value depends on the atmospheric pressure at the missile, an amplifier for said voltage, a synchro-generator having an exciting winding and an output winding, means supplying said amplified voltage to the exciting winding, a second similar synchro-generator, having a constant excitation, a free gyroscope carried by the missile and mechanically connected to said second synchro-generator to control the relative position of the excitin winding and the output winding of the second synchro-generator in response to the orientation of the missile, a transformer having a plurality of windings, a circuit including, in series, one of said transformer windings and the output windings of both synchro-generators, means for providing rectified power from a portion of the amplified voltage and feeding it into another winding of the transformer, and an amplifier havin input terminals connected to a third winding of the said transformer.

2. A servo system as defined in claim 1 wherein the means for producing an alternating voltage whose value depends on the atmospheric pressure at the missile includes an electrically heated resistor in heat-conductive relation with the atmosphere at the missile.

3. A servo system to be carried by a guided missile, comprising means for producing an alternating voltage whose value depends on the atmospheric pressure at the missile, an amplifier for said voltage, a synchro-generator having an excitin winding and an output winding, means supplying said amplified voltage to the excitin winding, a second similar synchro-generator having a constant excitation, orientation-responsive means carried by the missile and connected to said second synchro-generator to control the relative position of the exciting winding and the output winding of the second synchro-generator in conformity with the orientation of the missile, a transformer having a plurality of windings, a circuit including, in series, one of said transformer windings and the output wind ings of both synchro-generators, and means supplying unidirectional electrical power proportional to the atmospheric pressure at the missile and feeding said power into another winding of the transformer, to control the transformation ratio thereof.

JOHN H. BROADBENT. HENRY REES MITCHELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,793,213 Dowling Feb. 1'7, 1931 1,815,516 Lee July 21, 1931 2,159,142 Fischer May 23, 1939 2,306,578 Wetzel Dec. 29, 1942 2,311,118 Matthews Feb. 16, 1943 2,351,980 Lee June 20, 1944 2,387,795 Isserstedt Oct. 30, 1945 2,401,168 Kronenberger May 28, 1946 2,412,471 Olson Dec. 10, 1946 FOREIGN PATENTS Number Country Date 595.749 France July 24, 1925' 

